High volume mechanical ventilation (MV) induces a lethal pulmonary complication, termed ventilator induced lung injury (VILI), via the generation of toxic free radical and oxidant species that damage the alveolocapillary membrane, trigger pro-inflammatory gene expression, and induce pulmonary arterial hypertension and respiratory failure. To address this unmet medical need, we are developing a novel bifunctional small molecule, R-503, that: 1) prevents the generation of peroxynitrite via broad-spectrum redox catalysis, and 2) competitively inhibits the nuclear DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP- 1). In murine LD100 models of zymosan-induced multiple organ failure (MOF) and Cl2 inhalational lung injury, resuscitation with R-503 begun after the onset of zymosan instillation or Cl2 inhalation reduced neutrophil (PMN) infiltration by 65% and histologic lung injury by 80% (p<0.001). Based upon the centrality of inflammation and redox stress to the pathophysiology of VILI, we hypothesize that PARP-1 is highly activated in the lung and that injury will be attenuated after R-503 intervention. Specific Aim #1: Establish the pharmacodynamic (PD) profile of R-503 in a rat model of VILI Sprague-Dawley rats will be subjected to high tidal volume (TV) MV with positive end expiratory pressure (PEEP) of 3 cm H2O. TV will be initially set at 30 mL/kg and then reduced to limit peak inspiratory pressure (PIP) to 30 cm H2O. Treatment with R-503 (0, 10, 30, and 80 mg/kg) will be initiated 1 h after the onset of high volume MV. A negative control group will undergo low-volume ventilation (TV=6 mL/kg, PEEP=3 cm H2O). A positive control group will undergo high-volume MV plus therapy with IV methylprednisolone (30 mg/kg). A comparator group will be administered IV N-acetylcysteine (NAC) at an equivalent mass dose (30 mg/kg). Lung mechanics will be evaluated hourly, allowing for determination of: 1) dynamic respiratory system compliance, 2) resistance and hysteresivity derived from dynamic pressure volume loops, and 3) static compliance, including lower and upper inflection points derived from the shape of the static inflation and deflation airway pressure curves. We will monitor hemodynamic status by tracking heart rate, mean arterial pressure, and arterial pulse pressure variation. Arterial blood gas determination followed by bronchoalveolar lavage (BALF) of the left lung will be undertaken 4 h after the onset of MV, directly before euthanasia, allowing for determination of gas exchange and the PCO2/Minute Volume Ratio. The BALF will be analyzed for cellularity, LDH release, pro-inflammatory protein expression, and protein content. The right lung will be analyzed for histologic injury, wet/dry ratio, biochemical markers of redox stress (F2-isoprostane, GSH/GSSG ratio), PMN infiltration (myeloperoxidase; MPO), pro-inflammatory protein expression, and immunohistochemical evidence of peroxynitrite-induced 3- nitrotyrosine (3-NT) and poly(ADP-ribose) formation. Lung concentrations of R-503 will be correlated with outcome measures, so as to construct a PD profile.